ETTL is in the camera body, not the flash, the 580 is unlikely to make your exposures any better. What it is better for is ease of use, but the buttons, dials, and the one button head rotation thing. It looks nicer too.
OK,
The recycle time in almost instant and with a batterypack
for the flash by canon the flash rocks. I would recommend
the flash I have had excellent success !
I used to think that the upgrade was marginal until I realized how much faster the recycle time on the flash is... it is quite literally instant. The ergonomics are also far superior, and I will likely upgrade my remaining 550EX to a 580, as the operational particulars of the 580 are so much better. If you need a fast recycle, as I do for wedding and event work, there is simply no comparison!
With the 550 I was always concerned about exposure, but the 580s I have seem to nail it pretty much all the time.
The recycle time is much quicker, and the user inteface is much nicer too.
Would I sell a 550EX to buy a 580EX? That depends on you cash situation, and how much you would lose. I just bought some 580EXs, and use the old 550EX as a background slave flash when required.
